Magnetic UiO-66 functionalized with 4,4′-diamino-2,2′-stilbenedisulfonic as a highly recoverable acid catalyst for the synthesis of 4H-chromenes in green solvent

According to 4H-chromenes importance, we synthesized a novel magnetic UiO-66 functionalized with 4,4′-diamino-2,2′-stilbenedisulfonic as an efficient and reusable solid acid catalyst for synthesizing 4H-chromene skeletons via a one-pot three components reaction in a green solvent. The structure of the synthesized catalyst was confirmed by various techniques including FT-IR, XRD, BET, TGA, TEM, EDX, and SEM, and also the product yields were obtained in 83–96% of yields for all the reactions and under mild conditions. The reported procedure presents an environmentally friendly approach for synthesizing a significant number of 4H-chromene derivatives. Correspondingly, MOF-based catalyst makes it easy to separate from reaction media and reuse in the next runs.

advantages of both components, such as high chemical stability and simple separation process, for different applications, especially enhancements in the kinetics of adsorption [63][64][65][66] .
In detail, magnetic nanomaterials can act as effective adsorbents due to their ease of removing contaminants from wastewater by an applied magnetic field. Also, bio-sorbents have a synergistic effect with their efficient adsorption capacity to remove contaminants, to participate in waste minimization 67,68 , and to aid alleviate ecological complications 69 . For these reasons, resulted MOFs from these combinations possess interesting characteristics that could work adequately in CO 2 carbon capture. But the main disadvantage of using MOFs as adsorbents in CO 2 carbon capture is the energy-intensive nature connected with the desorption progression (sunlight, as a powerful external stimulus, can enable the desorption progression with much lesser energy demand over MOF materials). In these occasions, computational screening modeling approaches are influential tools to find optimum performing materials. With the aid of computational modeling, synthesized Mg-IRMOF-74-III showed a CO 2 adsorption capacity of 89.6 cm 3 g −1 , which is the highest CO 2 adsorption value within photo-responsive MOFs compared to the reported literatures 70 .
As mentioned above, due to magnetic nanoparticles' efficiency, the disadvantage of MOFs could vanish by various methods, like combining the MOFs and magnetic particles. Magnetic hybrid MOFs presented sizeable specific surface areas for their easy separation method. Several methods have been studied till today for the synthesis of magnetic MOFs. These methods include combining the MOFs with Fe 3 O 4 by a simple method, coating MOFs onto Fe 3 O 4 using layer-by-layer strategy, embedding Fe 3 O 4 into MOFs, and encapsulating Fe 3 O 4 into MOFs [71][72][73][74][75][76][77][78][79] . Among all the methods, synthesizing magnetic MOFs using a step-by-step method is one of the most reliable ways. The adjustability of the thickness of the outer shell MOFs is one of the main advantages of this method. Some adjustments are required to develop the compatibility of shell and core and gain the best results 80 .
Stilbenes are a class of secondary metabolites containing a trans/cis-ethene double bond and a phenyl on each of the double-bond carbon atoms. The majority of stilbenes are thermally-chemically stable. Additionally, they show fluorescence properties and absorption abilities 81,82 . They play in many required fields such as biomedical 71 , biophysical 67 , and photochemical research 63 . Due to their applications in a wide range of branches, stilbenes can be used in multidisciplinary fields and syndicates biology, medicine, physics, and chemistry together [83][84][85][86] . They are promising agents for use as a functional group for catalytic uses. As the other derivatives of aromatic sulfonic acids, stilbene sulfonic acids are also used to prepare optical brighteners and synthetic dyes 87,88 .
In this project, to investigate the applications of recoverable solid acid catalysts for the synthesis of 4H-chromenes, Zr clusters with 4,4′-Diamino-2,2′-stilbenedisulfonic were used to design modified magnetic MOF. Zr-cluster-based MOFs, like UiO-66 and UiO-67, have fascinating acid, thermal, and aqueous stabilities [89][90][91][92][93][94] . Due to their wide range of applications, we have synthesized 3) to study its application as a catalyst [95][96][97] in the synthesis of 4H-chromene skeletons via a one-pot three components reaction. The product yields were obtained in 83-96% of yields for all reactions. Studies showed that acidic reagent plays the main role in the catalytic cycle in these reactions.

Results and discussion
The Fe 3 O 4 @UiO@DAS catalyst was synthesized using a few steps presented in Fig. 2. Details of the preparation method are described in the experimental section.
The FTIR spectrum of Fe 3 O 4 (Fig. 4a), Fe 3 O 4 @UiO-66 can be seen in Fig. 4b. In this figure, the Fe-O band is appeared at 630 cm −1 (due to the presence of Fe 3 O 4 ), two peaks at around 1088 cm −1 are due to the presence of S-O (stretching vibrations), the peak at 1634 and 1709 cm −1 is attributed to C=C and C=O bands, respectively, the C-H bands can be seen at 2931 cm −1 , and the strong broad bands at 3435 cm −1 can be assigned to stretching of O-H (for Fe 3 O 4 ).
In the spectra of final product Fe 3 O 4 @UiO@DAS in Fig. 4c, in addition of mentioned peaks for Fe 3 O 4 @UiO-66, C-N peaks at 1502 and 1573 cm −1 can be seen. Also, aromatic peaks are appeared below 1000 cm −1 .
In the XRD analysis of Fe 3 O 4 @UiO@DAS (Fig. 5), observed diffraction peaks are similar to UiO-66 pattern which was reported before [98][99][100] . In this pattern, not any apparent variations in the characteristic diffraction pattern of Fe 3 O 4 @UiO-66 were observed. This shows that after growing on the surface of functionalized Fe 3 O 4 nanoparticles, the crystalline structure of the MOF was remained unchanged 99,100 .  www.nature.com/scientificreports/ To study the morphology, size, and also structure of Fe 3 O 4 @UiO@DAS, SEM and TEM analyses were used (Figs. 6,7,8). The SEM images can show the particle size (by randomly selected particles and studying the size distribution of them) and also illustrated that the particles have a cubic structure. TEM images of the prepared MOF show good agreement with other literatures and can confirm the Fe 3 O 4 core of the obtained catalyst. Additionally, using the EDX pattern of the synthesized catalyst, the main elements in its structure (Fe, O, C, N, S) could be understood. These analyses proved the successful synthesis of our catalyst. From EDX we can understand the different amount of carbon in our final catalyst (around 30 weights % in Fe 3 O 4 @UiO@DAS) from our initial samples.
TGA analysis shows the thermal stability of the synthesized catalyst ( Fig. 9). The first decomposition was placed between 100 and 200 °C, due to the trapped water. The second stage, between 260 and 330 °C, is attributed to the decomposition of 4,4′-diamino-2,2′-stilbenedisulfonic acid. Next, the other weight losses that occurred at around 350-390 and 400-420 °C, are because of the removal of hydroxyl, sulfonic acid, and carboxylic acid groups. In higher degrees, owing to the presence of Fe 3 O 4 , the line becomes stable with no considerable changes.
The N 2 adsorption-desorption data have been summarized in Table 1. The BET specific surface areas of magnetic Fe 3 O 4 @UiO-66 and Fe 3 O 4 @UiO@DAS are 828 and 725 m 2 g −1 , respectively.
After that, our synthesized catalyst Fe 3 O 4 @UiO@DAS was used. The best result was gained in the existence of 5 mg of catalyst (68% yield, reflux, 0.5 h) ( Table 2, entries 8-13). By studying the amount of catalyst, it was understood that in the presence of 13 mg of the catalyst, the yield of 94% could be gained at 30 min (Table 2, entries 15). By increasing the catalyst amount from 13 to 15 mg, no change in reaction yield was observed ( Table 2, entries 16). This result clearly shows that Fe 3 O 4 @UiO@DAS effectively improves the reaction yield. The acidic functional groups (SO 3 H) of 4,4′-diamino-2,2′-stilbenedisulfonic acid as Bronsted acids improve the reaction yield and the nano-particles can also race the reaction up as Lewis acids. However, the optimization Optimized reaction condition also was for the synthesis of 2-amino-7,7-dimethyl-5-oxo-4-aryl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile, which were collected in Table 4.
It is noteworthy that in all reactions for the synthesis of 4H-chromene derivatives (5, 6, 7) syntheses, the reaction of aromatic aldehydes which possessed electron-withdrawing groups are shown to be faster than the reaction of aromatic aldehydes with electron-donating groups. Dimedone required a shorter reaction time compared to the 4-hydroxy-pyrane and 4-hydroxy-coumarin.
For the importance of using heterogeneous catalysts in industrial processes, recyclability of our synthesized catalyst was studied using optimized reaction conditions for synthesis 2-amino-3-cyano-4H-chromene via condensation of benzaldehydes (1a), malononitrile (2), and 4-hydroxy-6-methyl-2H-pyran-2-one (3) in the presence of 13 mg of Fe 3 O 4 @UiO@DAS. Our gained results (Table 6) could be used 7 times without a dramatic decrease in its ability. After each reaction, the catalyst was separated by an external magnetic field and washed twice with hot deionized water (10 mL), once with 10 mL ethanol, dried in an oven at 60 °C for 24 h in a vacuum oven reused in the model reaction.    101,102 . After synthesizing it, Fe 3 O 4 @SiO 2 (1 g) was dispersed in dry EtOH and NH 4 OH (2 mL) was added to the mixture. Then, MPS (10 mL) was gradually added to the above mixture at 60 °C, and for 48 h the mixture was stirred. Using an external magnet, magnetic nanoparticles were collected, washed, and dried for 24 h under vacuum conditions. Next, 0.4 of synthesized magnetic nanoparticles (which we called MNP@MPS) was dispersed in MeOH (30 mL), and acrylic acid (0.4 g) was added to it. After purging Ar into the mixture (for 20 min), AIBN (0.1) was added to it and the mixture was stirred for 24 h at 70 °C. The final product (MNP@PAA) was collected by an external magnet, washed, and dried under vacuum conditions.
To synthesize of Fe 3 O 4 @UiO-66, synthesized MNP@MPS@PAA (0.2 g ) was added to DMF (30 mL) and sonicated for 30 min. Next, by adding 0.53 g of zirconium (IV) chloride (0.53 g) and terephthalic acid (0.38 g) to the mixture, it was left to stir for 2 h. after 2 h, the autoclave was used to heat the mixture (120 °C, 1 day).      Tables 3, 4, and 5. After reaction completion, which was controlled by Thin Layer Chromatography (TLC) test (using EtOAc/n-Hexane, 1:3 as solvent), the catalyst was separated by a magnet, and the obtained solid product was filtered. In the case of impurities, the obtained product was recrystallized from ethanol.

Conclusions
Zr-cluster-based MOFs have fascinating characteristics and have huge variety of applications. To increase their applications, hybrid nanomaterials based on MOFs have synthesized. Synthesizing hybrid materials make it possible to use the advantages of both parts in their structures. In this project, to use the advantages of MOFs (such as high chemical stability) and magnetic nanoparticles (such as simple separation process) we have decided to synthesize a novel magnetic UiO-66 functionalized with 4,4′-diamino-2,2′-stilbenedisulfonic. This modified MOF characterized by various techniques, including FT-IR, XRD, BET, TGA, TEM, EDX, and SEM. To investigate the applications of our modified magnetic MOF, it was used for the synthesis of 4H-chromene skeletons via a one-pot three components reaction in a green solvent. This non-hazardous, recyclable, effective, and appropriate catalyst allowed quick and effective access to diverse 4H-chromene derivatives. The synthesized catalyst can Table 1. N 2 adsorption-desorption data.

Sample
Total pore volume (cm 3 g −1 ) BET surface area (m 2 g −1 ) Pore diameter (nm) www.nature.com/scientificreports/ be extracted from the reaction media by an external magnetic field and recycled. Briefly, the absence of harsh conditions in the synthesis of catalyst, reusability, mild reaction conditions, and up to 96% yields of products are advantageous of our introduced method.